The invention pertains to the field of sampling very high frequency RF signals. More specifically, the invention relates to an integrated sample head which uses equivalent time sampling for generation of an intermediate frequency output signal which is an equivalent time replica of the RF input signal to be sampled.
In the prior art, thin film signal samplers have been made in several different forms. In the earliest form, the signal samplers were brass blocks with holes machined therein with suspended center conductor to act as waveguides. One hole was used as a waveguide for the RF signal to be sampled and another hole was used to guide the sample pulses used to turn on the sample gate. The difficulty with these sampling devices was that the frequency at which they could operate was limited by the smallest diameter hole which could be machined into the brass block. The diameter of each waveguide defines the upper frequency at which the waveguide was useful.
A later version of prior art signal sampler design involved hybrid assemblies of discrete components. In this type of device, discrete diodes and thin film quartz substrate technology with integrated planar waveguides was used. U.S. Pat. No. 4,672,341 is representative of this technology. The difficulty with this approach was that integration on the substrate was on both sides with an integrated waveguide on the front side bringing the sample pulse in and an integrated waveguide on the back side bringing in the RF signal to be sampled. The structure of the device also involved a third layer microstrip. The RF signal to be sampled was guided through a via hole to the diodes on the top side of the substrate. This via hole caused extra parasitic inductance in the signal path and put a limit on the bandwidth for the signal sampler.
Performance of these hybrid structures was limited by how well the three layer structure could be fabricated and aligned as well as the intrinsic limitations caused by the spatial separation of the sample pulse generating structure from the sampling diodes. At frequencies in the range of hundreds of GHz, with signals traveling at approximately one-third the speed of light along the waveguides, even the smallest spatial separation between devices can cause losses and dispersion, and results in parasitic components which limit the bandwidth. Therefore, even a 100 micron misalignment in fabrication of such a structure translates to a one picosecond penalty. Since the desired aperture time is less than 5 picoseconds for a large bandwidth, such misalignment errors can substantially adversely affect the bandwidth by rendering it impossible to get a fast edge to the differentiator to generate a very short sample pulse. To get a fast edge to the differentiator, it is necessary to have very close spatial proximity between the structure which generates the short sample pulses and the signal line carrying the signal to be sampled.
An attempt at improving the hybrid structure is found in the latest generation of Hewlett Packard's signal samplers dating from February of 1986. In this latest generation of signal samplers, integrated gallium arsenide diodes are used for sampling the RF signal. Beam leads couple these diodes to step recovery diodes which generate the sample pulses which turn the gallium arsenide diodes on. The pulse generator, however, is not integrated on the same substrate with the differentiator because prior to the invention described herein, it is believed that no workers in the art were in possession of a gallium arsenide pulse generator of a monolithic design. In the Hewlett Packard design, integrated diodes, resistors, and capacitors are formed on the gallium arsenide substrate. These integrated components are connected by beam leads bonded to pads on the substrate to make connection to the other elements of the sampler circuit. The nonintegrated structures are a differentiating line to differentiate the voltage steps from the step recovery diodes to generate the sample pulses used to turn on the gallium arsenide diodes, and a microstrip line integrated on another substrate. Thus, the Hewlett Packard design requires at least two substrates with connections between them. This spatial separation between the pulse generation circuitry and the sampling diodes causes losses, dispersion, and parasitics which limit the bandwidth of the sampler.
It is highly desirable in many applications to work with RF signals having very high frequencies such as 300 GHz. To be able to see these signals on low frequency oscilloscopes for analysis, testing, and other purposes, it is necessary to down convert them to a lower frequency. One way of doing this is to sample these signals to generate a replica signal at a lower frequency which is within the range of frequencies which can be observed on commercially available oscilloscopes. To do this requires a very wide bandwidth signal sampler which generate sample pulses to turn on the sampling diodes having a pulse width on the order of less than 2 picoseconds. This requires precise control of dimensions and close proximity of all elements such as can be obtained in fully integrated, planar structures.
Therefore, a need has arisen for a signal sampler with a fully integrated monolithic design wherein spatial separation between the pulse generator and the sampling diodes is minimal, and wherein dimensional controls can be very exact.
Also a need has arisen for a more compact integrated nonlinear transmission line structure. This structure must have a characteristic impedance which is high enough to yield an overall characteristic impedance of approximately 50 ohms when loaded by a plurality of Schottky barrier varactor diodes.